I have a MapService published to ArcGIS Server 10.6.1. The data comes from a local FGDB.
In the MXD when authoring the document to define the service I set the aliases:
Because aliases in the FC in FGDB are not good:
Field properties set in ArcMap are exposed through the feature service; this includes field aliases
But I could not find any information about this in MapSerivces.